As used herein, the term "auxiliary stream" refers to a stream generated during the recovery of .epsilon.-caprolactam which does not contain primarily .epsilon.-caprolactam or poly .epsilon.-caprolactam.
As landfills continue to reach capacity, raw materials are depleted, and man recognizes that the earth's resources are limited, more and more materials are recycled. Synthetic polymers have long presented problems in recycling due to their often being commingled with other materials and to sometimes apparently irreversible polymerization from which useful raw materials cannot be easily obtained.
Certain polyamides, however, are known to be hydrolytically convertible to monomers which can be re-used. Especially in the case of nylon 6, the monomeric starting materials are reclaimed from waste polymer and used in the manufacture of man-made fibers. The literature reveals procedures for reclaiming such monomers and polymers. L. A. Dmitrieva et al., Regeneration of .epsilon.-Caprolactam from Wastes in the Manufacture of Polycaproamide Fibres and Yarns, Fibre Chemistry, March 1986, pp. 229-241, describes methods for reclaiming polycaprolactam (nylon 6) waste.
There are generally two methods for reclaiming nylon 6 waste. The first involves reprocessing the waste nylon 6, for example, via remelting and extrusion, to form useful articles. This concept is demonstrated in U.S. Pat. No. 4,143,001 to Raab et al.
The second method involves chemical regeneration through depolymerization. Processes for depolymerizing solid polyamide waste are known. For example, U.S. Pat. No. 2,343,174 to Edison et al. shows general hydrolytic depolymerization using steam. U.S. Pat. No. 3,988,406 to Nakamura et al. shows the recycling of polyamide waste by thermal depolymerization.
Among the polyamides depolymerized for re-use of the monomer is nylon 6. For example, U.S. Pat. No. 4,107,160 to Dicoi et al. describes reclamation of solid nylon 6 waste (generated during the processing of nylon 6), low molecular weight oligomers and residual monomer from the polycondensation of caprolactam.
Other polymers are also recycled. An example of a process for continuously degrading various plastics is provided in U.S. Pat. No. 4,051,212 to Grigat et al. Grigat et al. shows a process for continuously hydrolytically degrading plastics. The hydrolyzable material is introduced with water into a screw machine, where it is subjected to a temperature of 100.degree. C. to 300.degree. C. at a pressure of 5 to 100 bars for 2 to 100 minutes.
Although the motivation for reclaiming raw materials from waste polymer or spent polymeric products is well recognized, some products do not readily lend themselves to recycling. Items which are composites of several materials present problems. Along these lines, polymeric materials formed into carpets present an interesting reclamation problem. This is due, in part, to the variety of materials present in traditional carpet and the manner in which they are intimately connected. In traditional carpets, the tufts are often nylon 6, while the backing of a nylon 6 tufted carpet may include jute, polypropylene and latex, among other things. Also, the latex may contain fillers such as calcium carbonate, clay or aluminum trihydrate. The chemical and physical nature of these materials is such that reclamation of .epsilon.-caprolactam from nylon 6 carpets has traditionally been considered too complex, too expensive and too cumbersome to be practical.
Traditional thought was that polypropylene or jute and especially latex would generate impurities which would make purification so difficult or the reclaimed yield so low as to render depolymerization of carpets impractical. Moreover, the CaCO.sub.3 usually present in the latex as filler would neutralize an equivalent amount of any acid depolymerization catalyst, such as H.sub.3 PO.sub.4.
In addition, many recycling schemes focus on a single ingredient in a mixed waste stream, leaving the remaining ingredients for landfilling, incineration, etc. Yet, the more portions of mixed waste that can be regenerated to materials having another useful life, the more valuable, economically and environmentally, is the process.